Southampton research helps to challenge the cause of halting star formation in massive galaxies

Southampton research helps to challenge the cause of halting star formation in massive galaxies

Massive galaxies could actually generate a sudden increase in star formation just before they shut down completely, according to new research by a team of astronomers including researchers from Physics and Astronomy at Southampton.

Until now, most data have shown clear evidence that star formation in galaxies steadily declines with time. The cause of this halting star formation is an ongoing and continuously debated mystery.

Now the new research conducted by Southampton, the Institute of Astrophysics of the Canary Islands and the Institute of Space Sciences, has revealed that galaxies with active cores may undergo a period of rapid star birth before they take their last breaths. The findings have been published in the Monthly Notices of the Royal Astronomical Society: Letters.

Professor Francesco Shankar, Professor of Astrophysics, who was involved in the research said: "Local galaxies are observed to gradually reduce the rate in which they are forming stars, but our data suggest that supermassive black holes are not the major culprit here. They may contribute to quenching star formation, but the quest is still open."

Most, if not all, massive galaxies are believed to harbour supermassive black holes at their centres. When supermassive black holes are actively accreting gas, they can radiate as much energy as the galaxy in which they reside. The popular picture is that powerful winds and jets produced by the central Active Galactic Nucleus (AGN) are able to expel or heat gas within the host galaxy, removing the material needed to make stars.

However, the study suggests that galaxies with an AGN hosting continuously growing black holes that emit large amounts of energy and radiation have temporarily increased the rate at which they form stars by accumulating fresh gas from their surroundings.

Equipped with a large sample of more than three thousand nearby galaxies hosting an AGN, the team used new data analysis techniques to measure how the star formation of the galaxies has changed over cosmic time.

Challenging the expectations from the standard scenario, the team found that nearby galaxies hosting AGN are not simply gradually quenching their star forming rate. On the contrary, they are going through a "rejuvenation phase" most of the galaxies in the sample with active black holes show clear signatures of a rising not declining star formation rate in recent epochs.

The team concluded that current AGN activity may be contributing to, but not causing, the observed quenching of star birth. Therefore, the rejuvenation of galaxies with active nuclei may be the "last breaths" in the lives of galaxies, before they completely stop forming new stars.

Francesco is a theoretical astronomer renowned for his work focused on the statistical description of the cosmological evolution of supermassive black holes. He contributed to the present research suggesting new analyses of different data sets and then guiding the interpretation of the results.